Grass Valley PDR v.2.2 - Manual (Page 23)

Video Compression
Profile Family 5
Video Compression
The i960 on the master board is used to control data flow and compression
coefficient loading of the JPEG CODECs. The amount of video compression
varies according to the setting of the compression coefficient. Higher
compression ratios store more data, but the result is lower quality video. On the
other hand, lower compression ratios result in higher quality video and less
storage capacity. Audio is not compressed.
Since JPEG compression ratio can be varied to change the video quality given
available storage time, the amount of storage depends on your choice of
compression ratio. A quick rule of thumb is that five minutes of video—plus
four channels of audio and two channels of timecode—is roughly equal to one
gigabyte of disk storage at a 24 Mbps compressed video data rate. For example,
a PDX208 Disk Expansion unit expands storage up to twelve hours and a
PRS200 RAID Storage System can brings it up to approximately 96 hours.
In addition to video compression, the recorder boards also integrate the audio
data, coming from the EISA bus, with compressed video data. with up to four
channels of audio per channel of video. These recorder boards communicate
with the SCSI-2 interface using a Direct Memory Access (DMA) interface.
Video data is written to and read from the disk over a fast/wide/differential or
Ultra SCSI interface. A maximum of eight 9 gigabyte disk drives are used for
storing the video and audio. Data is striped across disk drives for increased
aggregate bandwidth. External disk drives may be added, such as the PDX208
Disk Expansion unit or the PRS200 RAID Storage system. A maximum of
twelve drives can be supported on a single SCSI-2 bus. The master and slave
recorder boards each have one SCSI-2 interface.